chore: mvr 6.0.0 release prep

[sbansla]

Title: Drop Node.js 14/16/18 support, bump to 6.0.0

Body:

Summary

• Bump version to 6.0.0 (major version release)
• Raise minimum Node.js engine from >=14.0 to >=20.0
• Update CI test matrix from [14, 16, 18, 20, 22, lts/] to [20, 22, 24, lts/]
• Update GitHub Actions in deploy job: actions/checkout@v4, actions/setup-node@v4, docker/login-action@v3
• Update README.md supported versions: Node.js 20, 22, 24
• Add UPGRADE.md entry for 5.x.x to 6.x.x migration

Test plan

• CI passes for Node.js 20, 22, 24, and lts/*
• Unit tests pass on all matrix versions
• Cluster tests pass on all matrix versions

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[YasharF]

Heads up:

• Nodejs 20 is going to be EoL next week on 30 Apr 2026 https://github.com/nodejs/release#release-schedule
• Nodejs 26 is going to be released also next week on 28 Apr 2026 2026-04-28, Version 26.0.0 (Current) nodejs/node#62526 (comment)